Potentiometer assembly



Aug. 1948. o. J. UNDERWOOD ETAL 7 2,447,780

POTENTIOMETER ASSEMBLY Filed May 5, 1947 12 E4 9 la 1/ &

Zmventoxs OITVMLE J UNOE/f WOOD (Ittomeg Patented Aug. 24, 1948 UNITED STATES PATENT OFFICE neapolis, Minn.,

assignors to Minneapolis- Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application May 3, 1947, Serial No. 745,862

13 Claims.

This invention relates to an improved form of potentiometer for electrical regulating systems and other comparable applications, and it has particular relation to means for increasing the number of steps of resistance adjustment which such potentiometers are capable of providing. It is therefore an object of thi invention to provide a potentiometer apparatus with a multiple fingered wiper arrangement which will increase the number of steps of resistance adjustment.

Multiple finger wiper arrangements are not new in the art of potentiometers, but their design has been directed towards a method of insuring that at least one wiper contact will be in electrical contact with a turn of wire of the resistance element of the potentiometer at all times so that the possibility of an open circuit at the potentiometer is thus eliminated. Such an arrangement, however, will not afiect an increase in the number of steps of resistance adjustment over the adjustment which such elements are capable of providing, regardless of the number of wipers and the manner in which they are spaced. It is an object of this invention to provide a potentiometer apparatus in which each of the multiple fingered wiper arms possess an appreciable amount of resistance between a contact tip for each of the multiple arms and a point common to all of the multiple arms of the wiper apparatus.

It is a further object of this invention to provide such an arrangement in which the increased number of resistance adjustments are constant throughout the entire range of the potentiometer.

It is a still further object of this invention to provide a multiple fingered wiper arrangement which can be used with all forms of wound resistance elements.

Various other objects, advantages and features of novelty which characterize our invention are pointed out with particularity in the claims annexed hereto and forming a part, hereof. However, for a better understanding of the invention, its advantages, and objects attained by its use, reference should be had to the subjoined drawing, which forms a further part hereof, and to the accompanying descriptive matter, in which we have illustrated and described a preferred embodiment of our invention.

Figure 1 is a schematic electrical diagram of a conventional potentiometer used as a voltage divider so that the steps of resistance adjustment of the potentiometer can be readily measured.

Figure 2 is a schematic electrical diagram of the improved form of potentiometer used as a voltage divider so that the steps of resistance adjustment of the potentiometer can be readily measured.

Figure 3 is a top view of the multiple fingered wiper assembly.

Figure 4 is a side view of this multiple fingered wiper assembly showing the insulation between the wiper arms.

Figure 5 is a series of schematic views of the contacts of the wipers as they pass from one turn of wire of the resistance element to another turn.

Figure 6 is a series of schematic views of the variable resistance combinations which are obtained as the contact tips pass from one turn of wire of the resistance element to another.

In Figure 1 is shown a conventional potentiometer l5 which comprises a resistance element 21 and a wiper contact arm 23. The resistance element 21 has terminals 1, and I3 which are connected across the terminals of the secondary winding l5 of transformer H. The primary winding l8 of transformer I1 is energized from any suitable A.-C. source (not shown). For the purposes of measuring the resistance or voltage adjustment to be obtained from this potentiometer, a voltmeter or galvanometer 2| is connected to wiper 23 and also to a midpoint 24 of the secondary winding l6 of transformer IT. With this arrangement, where the wiping contact 23 must pass from one turn of wire to another, it is found that the voltage variation as charted against the movement of the wiper 23 along the resistance element 21 results in a stepped voltage variation. As long as the wiping contact 23 is touching one specific turn of wire, a definite voltage prevails. When the contact 23 arrives at the valley or dip between the two windings and it, at that time, comes in contact with both windings, an instantaneous variation in voltage results which is so short-lived that it can be disregarded for practical purposes. This variation i caused by the shorting by this contact of the one turn of wire common to the contact tip. The wiper they be spaced laterally along the resistance element 21, the arrangement noted above would arise inthat only the contacts which are closest to either extremity of the resistance element will be effective in contacting the resistance element, and the result is that such an arrangement would give no increased steps of resistance adjustment over the present form.

In the wiper assembly as shown in Figure 3, a 15 ohm resistor is attached to each of the terminal lugs 4 I, 42 and 43. schematically the members 5i, 52, 53 are applied to this external resistance and to any internal resistance of the wiper arm and wiper contact as a unit, but if the wiper arms are designed to include the necessary resistance no external resistance is necessary. Here the free ends of these 15 ohm resistors are together in a common point 60. Potentiometer winding 21 is wound with #37 A. W. G. Nichrome wire having a total average resistance of 1280 ohms for 520 turns or an approximate resistance of 2.5 ohms turn. With this amount of external resistance added the wiper arms, the ratio be tween the resistances in the wiper arms 3 I. 32 and 33 to the resistance of the portion of resistance element 21 between the contacts 34, 35 or 35. 36 is approximately 6 to 1. With 30 volts applied to the'extremities 7, l3 of potentiometer winding 21, from secondary winding I6 of transformer l1, there is approximately a .0578 diiierence in voltage drop between each adjacent turn of wire. By using voltmeter 2| to measure this voltage variation which occurs as wiper 28 is moved across potentiometer winding, the actual number of steps is evident and the value of each of the resistance steps obtained can be determined. A milliammeter (not shown) inserted in circuit between the wipers and secondary tap in series with voltmeter 2| or millivoltmeter is necessary to complete this calculation. This arrangement was also tried with external resistances of three ohms and one hundred ohms, or approximately a ratio of l to 1 and 40 to 1 respectively with that resistance included between the contact tips. With three ohms added to each wiper arm the phenomenon of increased resistance adjustment steps became evident, but not until the external resistance was increased with these increased steps clearly defined. With one hundred ohms of resistance in each of the wiper arms. the steps were still clearly defined, but the rate of the change of the resistance steps were no longer uniform. The ratio which appears to give the most uniformity between steps is approximately 5 to l and a ratio greater or less than this will tend to vary the proportion of change between steps. Figures 5 and 6 include a series of sketches a, b, c and d which show the positions of the wiper assembly 28 with respect to the turns of the potentiometer winding immediately beneath the wiper assembly 28 as said wiper assembly is moved a distance equal to the width of one whole turn of resistance winding. For convenience in identifying the turns beneath and in contact with the wiper contacts of wiper assembly 28 during this movement, these turns are numbered 8-42 in sketches a-d of Figures 5 and 6. These reference numbers refer to the particular point on the individual turns of the potentiometer winding at which the cross section of Figure 5 is taken. The resistance shown in Figure 6 between these numbers indicates the value of the actual resistance of the turns between these points. Figure 6 is merely a series of schematic sketches of various resistance combinations occurring between the wiper assembly and that section of the potentiometer winding immediately in contact with the wiping contacts of said assembly as the wiper assembly 28 is moved through a series of positions with respect to the turns of the potentiometer winding 2'! over a distance of one whole turn. The axial movement between each sketch is equal to the distance of one third of the turn from each adjacent position. The sketches a of Figures 5 and 6 disclose the wiping contacts 34, 35, and 36 in contact with the turns numbered 8, 9, and II respectively. As noted above, these wiping contacts are spaced a distance of 1% turns apart. The actual value of the resistance combination shown in the sketch b of Figure 6 can be calculated by treating any two of the adjacent resistance arms 5!, 52. and 53 and the resistance of the potentiometer winding 21 between the points of contact of these two resistance arms as a delta circuit combination. By simple mathematical calculation, this delta circuit can be converted into an equivalent star circuit. Two legs of this star circuit can be combined with the remaining resistance arm and turn or turns. of resistance winding remaining in the network or combination to form another delta circuit which.

can similarly be resolved into a final resultant star circuit. In converting these resistance combinations from delta to star circuits the common point must be kept as a terminal point. The resultant star circuit when combined with the remaining leg of the first star circuit will give a single resultant star circuit which is the equivalent of the circuit resistance combination shown in sketch 6a. The two legs of the resultant star circuit common to the potentiometer winding, that is, the legs terminating in points 8 and II are effectively part of the resistance of the potentiometer winding 21. The third leg of the resultant star circuit which is the resistance between the common point of the star and the common point of the wiper assembly 28 is effectively the resistance of the wiper. Thus the actual resistance of the potentiometer winding includes those portions of the winding on either side of the wiper assembly and the beforementioned legs of the star circuit. In measuring the resistance of a portion of the winding between the common point of the wiper assembly 5!] and the extremity 1 or I3 of the winding 21, there will be included in the circuit that portion of the star circuit which is efiectively the resistance of the wiper. In using a measuring instrument or device with sufficiently high inherent impedance, the amount of voltage drop due to this resistance between common point of the resultant circuit and the connection to the measuring instrument is negligible and can be disregarded, or calculated if necessary. Similarly, sketch 1) shows the wiper contacts 34, 35 and 36 in contact with turns 8, Hi, and II respectively; sketch 0 shows these wiper contacts 34, 35 and 36 in contact with turns 9, Ill and II respectively; and sketch d shows these wiper contacts 34, 35 and 36 in contact with turns 9, ID and I2 respectively. Sketch d shows the position of the wiper assembly after it has been moved along the direction of the higher numbered turns a distance equal to the.

width of one turn of resistance wire of the winding 21. The resistance combination of the sketches b, c, and d of Figure 6 can be similarly evaluated as indicated above. When computed these resistance combinations, with the 15 ohm resistance in each wiper arm, showed an increase in the value of the resistance between one exmamas dainty of said combination and this common 15 tor the rsu iit staicircuit over each preceding resistance combination an amount which was approximated 33% of the change in the lstance between the same points during the total movement or the assembly a distance of dfle tfifi'dl tiiffi. By increasing Or decreasing this fiat/i6 which was approximately 6 to 1 when 15 earns was used in each wiper arm, the amount of variation between the values for each posithin; as the steps obtained; is changed and they are 'nd longer uniform although the number of steps still remain a fixed and definite number.

it routes that with four 'or five wipers, spaced piefgrably 1% and 1 turns apart, respectively, that four 6i five steps or resistance variations are available. This is, of course, eliminating and disregarding any variations which might occur when one bi wiper contacts shorts out a turn or a 'ii'uni'ber of turns of the resistance element 21. In the above illustration, teference to the effect of (inane-e in the value of resistance of thank) tehtiiheter winding 61' a portion thereof between one eitrniity am the ension point of the wiper assembly when one of the wipers shorts out a turn or the potentiometer winding 21, has been 6' ('1 since this occurrence is only instantameans and has no noticeable eifect on the operanot of such voItag'e or resistance varying devices.

in considering this invention it should be kept 1h jini'ii'a' that the present diselosure is intended to be illustrative only and the scope of this inven tion is t6 be'deter'rnined only by the appended c'lii We claim:

1. In 'a potentiometer, a resistance element cbinpiisihg a plurality of turns of resistance wire, a w'ip assembly including a plurality of contact members fmovable relative to said resistance elem'eiit in electrical Contact therewith, said contact Iii fnb'ers being laterally spaced alongsaid resistan element by a distance which is not equal to any whole number of turns; and said wiper assembly havin fr'esistance members connecting each arsaia contact members to a common junction point; the value of each of said members being greater than the smallest value of resistance of said resistance element between the points of engagement of any two of said contact members.

:2. In a potentiometer; a resistance element onipiisin a plurality of turns of resistance wire, a plurality of contact wiper memb rs each haviii'a'vvipfei' arm and a contact tip, means mountir'i together said contact wiper members as a unit for movement relative to said resistance ele- Ii M are is electii cal contact therewith, said wiiii arms being connected together at a commefisbim, said contact tips being laterally spaced along said resistance element by a distance which is" 'ot equal to any whdle number of turnssaid fintiict members having an internal resistance betwe ii the point of contact to said resistance elernent and the point common to the wiper arins which is greater than the resistance of the turns of were of resistance element between two adjacent contact tips; and means including said con tact wiper members for moving each of said contact tips laterally along the resistance element, the value of the portion of said resistance element between one extremity thereof and a common pfiint on the Wiper arms being varied by two or more stepped variations as said contact tips move adistance equal to width of one turn of wire exelusive of any resistance variation which occurs when any one or more of said'contact tips rno-' mentarily short oiit one or i'nore' turns of said re; sistance element.

3. In a device of the class described, in combination, a resistance element comprising a'plu rality of turns of resistance wire, a source of power connected to the extremity of said resistance element, a plurality of contact wiper members each having a wiper arm and a contact tip, said Wiper arms being connected together at a common point, said contact tips being laterally spaced along said resistance element by a distanc which is not equal to any whole number of turns and mounted for movement relative to said resistance element and in electrical contact therewith, said contact members having an internal resistance between the point of contact to said resistance element and the point common to the wiper arms which is greater than the resistance of the turns of wire of resistance element between two adjacent contact tips, and means including said contact wiper members for moving each of said contact tips laterally along the re: sistance element, a voltage drop being obtained between one extremity of the resistance element and the common point of the wiper arms which varies by more than two stepped voltage varia: tions as said contact tips move a distance equal to the width of one turn of wire exclusive of any voltage variation which occurs when any one or more of said contact tips momentarily short out one or more turns of said resistance element.

4-. In combination; a coiled resistance element comprising a plurality of adjacent turns of resistance Wire, a plurality of contact members movable along said element and spaced apart along the direction of movement by a distance which is difi'erent from the width of any whole number of turns and connections between each of said contact members and a common junction point, each of said connections including a resistance member having a resistance greater than that of said resistance element between the points of engagement' of two of said contact members.

5. In combination, a coiled resistance element comprising a plurality of adjacent turns of re"-.

s'istance wire; a plurality of contact members movable along said element and spaced apart along'th'e direction of movement by a distance which is equal to the 1 it turns where N is the number of contact members used, and connectidns between each of said contact members and a common junction point, each of said connections including a resistanc'e'having a resistance greater than that of said resistance element be: tween the points of engagement of two of said contact members;

6. In combination; a resistance element com prising a plurality of turns of resistance wire; a plurality of contact wiper members each having a wiper arm and a contact tip, said contact m'e'm-J bers being insulated from one another and mounted to move relative to said resistance element in electrical contact therewith; said wiper arms being connected together in a common point; means including said contact wiper members f'or'laterally spacing along said resistance element said contact tips; each of said contact wiper arms having an internal resistance between said contact tips and a point common to the adjacent wiper arm which is greater than the smallest amount of resistance of the resistance element between any two adjacent contact tips.

7. In combination, a resistance element comprising a plurality of turns of resistance wire, a plurality of contact wiper members each having a wiper arm and acontacttip, said contact wiper members being insulated from one another and mounted to move relative to said resistance element in electrical contact therewith, means including said contact-wiper members for spacing said contact tips laterally'along said resistance element a distance not'equal'to any whole-number of turns, an external resistance the value of which is greater than the smallest amount of resistance of the resistance element between any two adjacent contact tips connected in series with each of said wiper arms, and said opposite ends of said external resistances being connected together to form a common point for said contact wiper member.

8. In a potentiometer, a resistance element comprising a plurality of turns of resistance wire Wound around a core member, a plurality of contact wiper members each having a contact arm and a contact tip, said contact wiper members being insulated from one another and mounted to move relative to said resistance element in electrical contact therewith, means including said contact wiper members for laterally spacing along said resistance element said contact tips a distance equal to 1 turns where N is the number of wipers used, said wiper arms being connected together in a common point with each of said contact wiper arms having an internal resistance between said contact tips and a point common to the adjacent wiper arm which is greater than the smallest resistance of the resistance element between any two adjacent contact tips, and means including said contact wiper members for moving each of said contact tips laterally along said resistance element, the resistance between an extremity of the resistance element and a common point of the wiper arms being varied by two or more resistance variations as said contact tips move a distance equal to the width of one turn of wire exclusive of any variations which occur when one or more contacts momentarily short out one or more turns of the resistance element.

9. In combination, a resistance element comprising a plurality of turns of resistance wire Wound around a core member, a source of power connected to the extremities of said resistance element, a plurality of contact wiper members each having a contact arm and a contact tip, said contact wiper members being insulated from one another and mounted to move relative to said resistance element in electrical contact therewith, means including said contact wiper members for laterally spacing along said resistance element said contact tips a distance equal to 1 turns where N is the number of wipers used, said wiper arms being connected together in a common point with each of said contact wiper arms having an internal resistance between said contact tips and a point common to the adjacent wiper arm which is greater than the resistance of the resistance element between two adjacent contact tips, and means including said contact wiper members for moving each of said contact tips laterally along the resistance element, a voltage drop being obtained between one extremity of the resistance element and the common point of the wiper arms which varies by two or more voltage variations as said contact tips move a distance equal to the width of one turn of wire exclusive of any variations which occur when one or more contacts momentarily short out one or more turns of said resistance element.

10. In combination, a. resistance element comprising a plurality of turns of resistance wire, a

source of power connected to the extremities of said resistance elementracplurality of contact wiper members each havingqa wiper arm anda contact'tip, said contact members being insulated from one another and mounted to move relative to saidresistance element in electrical contact therewith, means including said contact wiper members for laterally spacing along said resistance element said contact tips a distance not equal tov a whole. number of turns, an external resistance the value of which is greaterthan the smallest amount of resistance of the resistance element between any two adjacent contact tips connected in series with each of said wiper arms, said external resistance having its free ends connected in a common point, and means including said contact wiper members for moving each of said contact tips laterally along the resistance element, a voltage drop being obtained between one extremity of the resistance element and the common point of the external resistances which varies by two or more voltage variations as said contact tips move a distance equal to the width of one turn of wire exclusive of the voltage variations which occur when one or more of the wiper contacts momentarily short out one or more turns of said resistance element.

11. In a potentiometer, a resistance element comprising a plurality of turns of resistance wire, a plurality of contact wiper members each havin a wiper arm and a contact tip, said contact members being insulated from one another and mounted to move relative to said resistance element in electrical contact therewith, means including said contact wiper members for laterally spacing along said resistance element said contact tips a distance not equal to a whole number of turns, an external resistance the value of which is greater than the smallest unit resistance of the resistance element between any two adjacent contact tips connected in series with each of said wiper arms, said external resistance having its free ends connected in a common point, and means including said contact wiper members for moving each of said contact tips laterally along the resistance element, the resistance between an extremity of the resistance element and the common point of the external resistances being varied by two or more resistance variations as said contact tips move the distance of one whole turn exclusive of any variations which occur when one or more contacts momentarily short out one or more turns of wire of the resistance element.

12. In combination, a resistance element comprising a plurality of adjacent turns of resistance wire, energized at its extremities by a source of power, a plurality of contact members movable relative to said element in electrical contact therewith to form a voltage producing device whose voltage output varies with movement of said contact members, said contact members being spaced apart along the direction of movement a distance which is greater than the width of one whole turn but is difierent from the width of any whole number of turns, connections between said contact members and a common junction point each of which includes a resistance element whose resistance value is reater than that of said resistance element between the points of engagement of two of said contact elements, and means for moving said contact members along said resistance element, said output from said voltage producing device varying by a plurality of voltage variations as said contact ezuma ao 1-2 that of said resistance e1ement.-. between. vthe) points of-(engagement "oi two. oflsaigcoritachmembers, and: means mrSmWing said lcontach menibers along said resistanceelement. the resistance of saidresistance element between an.,.e gbmm ity thereof and the. \common junctionflbeing varied by two. onmore. nesistancehvanialzionsz s said contact members. are. maved the. .d istancesof one turn. ORVILLE, UNDERWQQDA RICHARD .S. EEIGAL. 

